Thanks Pete, someone mentioned they'd used one of those Infrared thermometers to me the other day and I thought, I bet they're not cheap, but they are, i'll get one in case I need one again but think I've somehow sorted it......
I've got a gas water heater still connected up that I used to use before I got my calorifier and thought I'd see if that would allow hot water to the taps and it worked fine. I had to turn the stop cock on just before the gas water heater and turn the one off that goes from the calorifier through to the taps and don't know what turning them on and off has done but I ran my engine for an hour after that and now have piping hot water again! I don't really understand this still or how even if these were blocked had an airlock they would have still only let 45oC water through rather than 65oC.
Anyway i'll keep my fingers crossed!
Thanks all for your help!
PS: I changed the thermostatic blender valve yesterday and ran the engine but still had the same problem.